Captive rays usually dont need the substrate/driftwood for hiding, couse in most situations they are keept in a non-threatening environment anyway. So that
need is eliminatet, more or less. But hiding is of course not the only aspect. - Captivity in it self does not eliminate their need for
mental stimulation/exercise for the brain.
And i dont belive captive breed rays lose all of their instincts and needs - just becouse they are captive breed.
Fishes can get bored too - when represented to monotonous and artifical environments. Thats a fact, and i strongly belive that conditions like that is no good for them long term. Stingrays possess cognitive abilities (
http://www.practicalfishkeeping.co.uk/content.php?sid=2649). They are problem solvers, and does not belong in a tank without anything to shift -or through around.
I am not trying to start a big argue btw! I just hope i can make you rethink how you keep them...

i really think all those beautiful fishes deserve better!